Sodium-calcium exchange in excitable cells: fuzzy space

Article Abstract:

In diverse species, and a variety of cells within the human body, nature has chosen the level of intracellular calcium as one of the key cellular regulatory messages. Calcium plays a key role in neurons, epithelial cells, and visual photoreceptors. One of the mechanisms for keeping intracellular calcium at the proper level is the sodium-calcium exchanger. Since there are more sodium ions outside the cell than in, there is an energy potential which wants to push sodium across the cell membrane, just as the energy of water behind a dam wants to push it past turbines to the valley below. And just as the water rushing past the turbines produces useful energy, so the rushing of sodium past the sodium-calcium exchanger produces useful energy. In this case, the energy is used to push excess calcium out of the cell. For each three sodium ions which enter the cell, a calcium ion is pushed out. The sodium-calcium exchanger has always been thought to be a slow device, involved more in long-term, tonic events. In the April 20, 1990 issue of Science, two research teams present evidence that the exchanger is more active in the control of heart muscle contraction than had been previously thought. One curious aspect of the new findings is that they do not jive with calculations of the free diffusion of ions that have entered the cell. In order to reconcile the experimental results with what is known about sodium-calcium exchange, one must hypothesize that ions are not free to diffuse inside the cell. There must be a limitation to diffusion of roughly 10 nanometers (billionths of a meter) inside the cell membrane. Ions would hypothetically be free to diffuse within this narrow region, but not beyond. Although direct evidence for such a region has not been obtained, the results of the physiological experiments on sodium-calcium exchange suggest that the internal cytoplasmic architecture may be more sophisticated than previously thought. (Consumer Summary produced by Reliance Medical Information, Inc.)

Heart, Heart contraction, Sodium in the body, Sodium (Nutrient), editorial

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Calcium-sensitive cross-bridge transitions in mammalian fast and slow skeletal muscle fibers

Article Abstract:

Currently accepted models of skeletal muscle contraction describe the formation, breaking, and reformation of cross-bridges between the proteins actin and myosin. This cross-bridge formation pulls the proteins towards one another, resulting in the development of physical force. However, it is not known why fast muscle and slow muscle, which have the same basic contractile apparatus, differ in speed. A method has been developed which permits the observation of contraction under conditions which directly control the concentration of calcium, a key ion in the contraction process. The membrane of the muscle cells is dissolved away, leaving the underlying contractile proteins intact and directly accessible to experimentally controlled levels of calcium. Experiments demonstrated that the rate of formation of the force-producing cross-bridges is calcium-sensitive for both slow and fast muscle fibers. However, the rate of cross-bridge formation in fast fibers is three to eight times greater, depending upon the calcium concentration. Although the results do not preclude additional effects that may modulate speed of contraction in intact muscle fibers, they suggest that the basic differences lie in the formation of cross-bridges between the contractile proteins themselves. (Consumer Summary produced by Reliance Medical Information, Inc.)

Striated muscle, Muscle contraction

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Muscling transplants into mice

Article Abstract:

Scientists have long sought something that would prevent the immune system from rejecting new organs. Certain cells in the body will not reject transplanted tissue. Muscles cells produce a chemical called FasL which has the ability to restrict immunosuppression to the area of the new organ.

Research, Prevention, Graft rejection, Immunosuppressive agents, Transplantation immunology

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